The present invention relates generally to the field of backup and standby power systems. More specifically, the present invention relates to an automatic transfer switch system that controls switching of power from a main source (utility power) to a backup source (generator) by using a state machine controller and low cost relay switches housed in a single electrical panel such that the transfer switch system is well suited for residential use and is less expensive and easier to install than conventional automatic transfer switches.
Transfer switches are well known for controlling the switching of power from a main source or a secondary or backup source. Most transfer switches have been developed for the very demanding challenges of switching power between sources of more than 10,000 Watts (1OKW). These types of high power transfer switches are usually expensive systems that use multiple electrical panels to implement their function. One panel might house the relays or switching devices that select power from either of two sources for delivery to a load. Another panel might house a microprocessor and associated logic necessary for controlling the switches and providing a user interface. The power that is delivered from the switching device is normally then routed to a distribution panel that provides circuit breaker protection. These high power transfer switch systems are costly to manufacture because of the multiple panels, power switching devices, and complicated logic panel. Often, very quick switching times are needed, which require the use of high cost static switches. In addition, professional installation is required to install such a system adding to the associated costs. This type of high power transfer switch system is generally impractical for the average residential low power application requiring less than 10 KW standby power. For a good background on the issues involved in selection of automatic transfer switches, reference is made to xe2x80x9cGuidelines and Practices for the Selection of Automatic Transfer Switches to be used in DOE Backup Power Suppliesxe2x80x9d, DOE Backup Power Working Group, Apr. 20, 1995.
For low power transfer switches intended primarily for residential use, it is most common to utilize a manual switch mechanism. These manual systems typically are designed into a single panel. This panel allows the user to select where power is delivered and from what source it will be supplied. Because the process is manual, it requires the user to toggle switches on and off to turn non-critical loads off and turn critical loads on. In the case where the secondary or backup power source is a generator, the user also is required to start the generator either from the panel or at the generator itself. Typically, manual transfer switches also require professional installation because utility power must be disconnected to allow for installation of the manual transfer switch between the utility meter and the fuse box or circuit panel.
When the operation of a low power transfer switch is automated, the most common approach has been to utilize some form of mechanical switch mechanism that drives a lever that turns off one circuit breaker and then turns on another circuit breaker. Such mechanical switch arrangements are well suited for residential use because they are less expensive and because the mechanical nature of the switching provides an inherent protection against simultaneously connecting the engine generator and the utility power. In a residential application, there is generally no expectation that power will be continuously supplied during the switching from utility power to the generator. As the time required to start an engine generator is significantly longer than the time associated with the operation of the mechanical switch, there is no motivation to use expensive quicker switching arrangements as part of the transfer switch. While various mechanisms for moving the mechanical switch mechanism can be used, a typical example is the use of a solenoid that moves the mechanical switch through a sequence of positions. The rule that a transfer switch must break the first connection before making the second connection is accomplished by insuring that the mechanical switch turns off the first circuit breaker before turning on the second circuit breaker. Examples of this type of electrically operated, mechanically held automatic transfer switches are the ASCO(copyright) Series 300 and ASCO(copyright) Series 940 transfer switches. These automatic transfer switches have microprocessor controls and can be connected and programmed to start an auxiliary engine generator.
Instead of using a microprocessor to control the automatic transfer switch, some control systems have been implemented using relays and delay timers to control the mechanical operation of a transfer switch. U.S. Pat. Nos. 4,189,649, 4,204,128, 5,892,297 and 5,903,065 describe examples of such control systems that use relays and delay timers. Relays are also used in an automatic bus transfer device as described in U.S. Pat. No. 5,939,802 that controls both single phase and multiphase power systems.
One of the problems with using relay switches in an automatic transfer switch is the need to provide short circuit and continuous over current protection. Relay systems that close contacts into a shorted circuit risk welding of those contacts. All relay systems are susceptible to welding of contacts. For example, if the relay contacts that switch the generator to the load are welded, then removing the energizing source from this relay will not allow the contacts to open. This means the generator is still connected to the load even though the energizing source for the relay system has been removed. If the utility power relay is energized to conduct utility power to the load in this situation, the utility power is now short circuited into the generator. One way to protect relay systems is to provide fast blow fuses; however, these fuses must be replaced when they blow.
Although significant advances have been made with respect to high-power automatic transfer switches, it would be desirable to improve upon existing low-power automatic transfer switches, particularly automatic transfer switches that are intended for residential use.
The present invention is an automatic transfer switch system that controls switching of power from a main source (utility power) to a backup source (generator) by using a state machine controller and low cost relay switches housed in a single electrical panel. The low power automated transfer switch system is well-suited for residential use and is less expensive and easier to install than conventional automatic transfer switches. The key to a low cost transfer switch is delivering power with low cost relays. This is accomplished by employing relay drive techniques that enable these devices to survive rigorous standards compliance testing. Preferably, the state machine controller measures utility and generator source compliance using an optical coupling arrangement, rather than a transformer. The automated transfer switch disconnects the utility and generator means through a circuit breaker providing for fast protection. The relay drive techniques disclosed provide a form of an electrical interlock. This is a key feature when seeking compliance standard approval. Essentially, an automated transfer system should not continuously be capable of backfeeding power from generator to utility, and the electrical interlock of the relays as taught by this invention prevents any such backfeeding.
In a preferred embodiment, the automated transfer switch has a single electrical panel that houses a first circuit breaker connected to the primary source, a second circuit breaker connected to the backup source, and at least a third circuit breaker connected to a load in the residence. A pair of electrically interlocked power relays are electrically connected between the at least third circuit breaker and the first circuit breaker and the second circuit breaker. Control logic is operably connected to the power relays and includes a state machine controller to control the operation of the power relays to switch from the primary source to the backup source in the event that the control logic detects a decrease in at least one measurable characteristic of the primary source greater than a predetermined value. Preferably, the state machine controller is a field programmable gate array (FPGA) and the first circuit breaker and the second circuit breaker are rated for less than 10 kilowatts. Preferably, the control logic provides control signals to start and stop the engine generator and monitors at least one measurable characteristic of the backup source after it is started before switching from the primary source to the backup source. A simple to operate operator panel is positioned on a front of the electrical panel.
The present invention is also directed to a method of operating the low power automated transfer switch and to a method of installing the lower power automated transfer switch.